This character was originally used in Japanese to mean (ornamental) hairpin, and was read with a kun'yomi of kanzashi. The similarity in shape between a hairpin and the weapon called a sai later led to this character being used to refer to the weapon. In modern Japanese, this character is read primarily with its on'yomi of sai, and is used to refer to the weapon. The hairpin sense and kanzashi reading are more often spelled 簪.

Originally a compound of 髪(kamu, “hair, hair of the head”, Old Japanese combining form) +‎ 挿し(sashi, “sticker-through (something that sticks through)”, the 連用形(ren'yōkei, “continuative or stem form”) of verb 挿すsasu “to stick through, to insert”).[1][2] The sashi changes to zashi as an instance of rendaku (連濁).

Alternately, this may be a compound of 髪(ka, “hair”, apophonic form of 毛ke “hair”) +‎ 挿し(sashi, “sticker-through”).[1]

A folk etymology occasionally encountered is that this as a compound of 花(ka, “flower”) + 挿し(sashi, “sticker-through”), as many decorative hairpins have floral themes. However, this etymology would mix the Chinese-derived on'yomi of ka and the Old Japanese-derived kun'yomi of sashi, which seems less probable than the above two possibilities.